1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an image forming apparatus, and more particularly, to an image forming apparatus that transfers a toner image formed on an image carrier onto a first endless belt before transferring the toner image onto either a second endless belt directly or onto a recording medium on the second endless belt.
2. Discussion of the Related Art
In electrophotographic image forming apparatuses, changes in temperature, humidity, or other environmental changes can change characteristics of toner and/or other components of the image forming apparatus, causing unwanted fluctuations in output image density (hereinafter also referred to as image concentration), which is an amount of adhered toner per unit area of a recording medium.
Several techniques have been proposed to reduce the changes in image concentration caused by such environmental changes.
For example, in some related-art image forming apparatuses, a toner image formed on an image carrier according to image data transmitted from an external personal computer or the like is transferred onto a recording medium conveyed by an endless transfer belt. In addition, at given time intervals determined, for example, by number of sheets, etc., a pattern image including a toner image having a given shape is formed on the image carrier and transferred onto the surface of the endless transfer belt to enable a reflection-type photosensor to detect an amount of toner adhering to the toner image of the pattern image. Image forming conditions such as development potential, light intensity for writing a latent image, and the like are then adjusted based on the detection results obtained by the reflection-type photosensor. Such adjustment can provide a constant image concentration.
In other related-art image forming apparatuses, a toner image formed on an image carrier is transferred onto a first endless belt or an intermediate transfer belt, and the toner image on the intermediate transfer belt is then transferred onto a recording medium conveyed by a second endless belt or by a sheet conveyance belt. A drawback of such a configuration is that, if the thickness of the intermediate transfer belt and/or the diameter of a roller for driving the intermediate transfer belt vary due to environmental changes, an average speed per rotation of the intermediate transfer belt can be affected as well, causing distortion of the toner image transferred onto the intermediate transfer belt.
To prevent such a problem, the intermediate transfer belt is provided with scales having marks each of which is located at a given pitch along the intermediate transfer belt, so that a sensor can detect the average speed of movement of the intermediate transfer belt based on the time interval between the passage of successive marks past a given point. According to these detection results, the driving speed of a motor that drives the intermediate transfer belt can be adjusted, thereby keeping the intermediate transfer belt moving at a given average speed.
In addition to the above-described detection method, the average speed of movement of the intermediate transfer belt can be detected based on a speed of rotation of a driven roller that extends the intermediate transfer belt. Yet another known detection method involves a pattern image consisting of multiple toner images arranged at predetermined pitches that is transferred onto the intermediate transfer belt. The average speed of movement of the intermediate transfer belt can then be detected based on the time interval between passages past a given point of successive multiple toner images of the pattern image formed on the intermediate transfer belt.
However, such an image forming apparatus including the intermediate transfer belt that moves at a predetermined average speed cannot prevent distortion of an output image. Even if the intermediate transfer belt moves at a predetermined average speed, when a sheet conveyance belt moves at a speed different from a predetermined or reference speed because of a change in diameter of a roller supporting the sheet conveyance belt, the toner image may still be distorted during transfer from the intermediate transfer belt onto a recording medium conveyed by the sheet conveyance belt. Such image distortion can also occur when a toner image is transferred from the first transfer belt onto the second transfer belt.
Further, when a toner image transferred from the image carrier onto the intermediate transfer belt is further transferred onto a recording medium, even if the image forming conditions are adjusted as described above, an output image to be formed on the recording medium cannot acquire a preferable image concentration because the concentration of the output image can become insufficient if a transfer failure occurs and a large amount of transfer dust is produced in a subsequent transfer process.